An industrial robot is provided with a robot wrist attached to the extremity of the robot arm and a robot hand attached to the extremity of the robot wrist and is adapted to perform various robot actions for particular work such as holding, transferring and/or assembling. In order to enable such an industrial robot to carry out such robot actions, an industrial robot is constituted so as to enable the movement and the positional change of the robot hand within a space by giving a degree or degrees of freedom of motion to the robot arm and the robot wrist respectively, and to enable a robot hand to perform holding and other work by the use of its own degree of freedom of motion. Accordingly, driving sources for driving the robot arm and the robot wrist are built into the industrial robot. The driving source of the robot wrist, in particular, is an electric motor or the like mounted on a part or the base end of the robot arm, the rotative motion of which is transmitted through a motion transmitting mechanism disposed within the robot arm to the robot wrist to drive the robot wrist through a motion transmitting mechanism disposed within the robot wrist to enable a tilting motion or swivel motion thereof. It is conventional to store a control program in the robot control unit beforehand and to give commands to the driving source, i.e., the electric motor or the like, according to the control program, in order to control the tilting motion or swivel motion of the robot wrist in a manner appropriate to the objective robot motion. In such a control system, the control program usually includes a so-called overtravel preventing program as well as a motion control program to restrain uncontrolled motion and to restrict the tilting motion and swivel motion of the robot wrist within a predetermined range of motion. However, it is necessary to provide a mechanism capable of detecting the virtual excessive motion of each part of the wrist over the predetermined range of motion quickly and interrupting the uncontrolled motion immediately, in addition to the overtravel preventing measures of a software system, for the security of the robot and for preventing damage to the wiring and the piping arranged within the wrist. Furthermore, in the wrist mechanism of an industrial robot adapted to be driven by an electric motor, it is usual to return each working part of the wrist mechanism always to the datum position of motion upon the connection of the power source to the industrial robot, and desired robot motions are then carried out after the position of each working part of the wrist mechanism has been confirmed. Accordingly, such an industrial robot is provided with so-called near-zero detecting means to detect the datum position or origin of motion.